Large trucks have been used for years as an efficient way to transport large and heavy loads from one location to another over roads and bridges. To reduce the wear inflicted upon roads and bridges by the transport of such loads, government regulations limit the weight of these loads. The regulations have been based on the longitudinal distance between sets of wheels, particularly between the front and rear wheels of the vehicle. Load size can be legally increased by increasing the distance between the front and rear wheels of the vehicle. With the desire to carry the maximum possible load, trailing tag axle assemblies have been added to vehicles to legally increase load capacity by increasing the effective length of the vehicle.
Trailing tag axle assemblies previously made and currently in use are generally attached to the vehicle in a manner intended to be permanent (i.e. bolted or welded directly to the vehicle). Because these assemblies are not intended to be removed from the vehicle, the assemblies are generally made to move between a raised and lowered position, usually by the means of hydraulic actuators or similar motion inducing apparatus, to stow the assembly on the vehicle when not in use. This allows them to be lowered to an operating position when needed and raised to a storing and inactive position when not needed or, when dumping a load.
There are several disadvantages to the trailing tag axle assemblies previously used. One disadvantage is that with the trailing tag axle generally being of a non-detachable nature, it is still attached to the vehicle when not in use. Some materials hauled by vehicles use all available cubic volume in a cargo portion of the truck without maximizing the weight load that can be legally carried, thereby obviating the need for the trailing tag axle. In these situations the attached trailing axle adds unnecessary weight to the vehicle and can increase the wear on the vehicle, such as the tires, or may interfere with desired vehicle operations.
The previous tag axle assemblies can be cumbersome when disengaging a load by interfering with the operation of a tailgate or other door assembly. Many times the axle assembly is stored over the rear door of the truck body and therefore interferes with raising the gate. This is particularly true with a high-lift gate that is hinged toward the front of the dump body to raise it high over the load. This is important when large rock, riprap or the like is dumped. It can also interfere with the accuracy of placing a load by being between the vehicle and the desired unloading location (e.g. a hopper on an asphalt paving machine).
Another disadvantage of previous tag axle assemblies is that in the stored (stowed) position the tag axle assembly may cause the overall height of the vehicle to be greater than governmental regulations allow.